Well that was a rigmarole! I got it to work using internet exploder only the link on Northern explorers web-page just takes you to the main pay pal site, not the paypal checkout page.
However I got around that by copying the link I had in Chrome on the pay bit into explorer and it worked finally!
So it looks like it's Chrome that is not working, when no paypal account.
I have detailed the steps I took here, it should help others:
1) Open up the northern explorers forum in your normal browser (Chrome or whatever you have been trying it with) and click the "buy tickets" button.
2) Don't bother entering any details, select everything in the address bar (the https:\\... text) at the top and press Ctrl + C.
3) Open internet explorer and paste that address into the address bar there by selecting the text and pressing Ctrl + V
4) Fill in the details now and press pay and hopefully the bloody thing will work.
Please pass this on to your mates who are having trouble.